1. Field of the Invention
The present invention relates generally to aqueous ionomeric gels having a high viscosity, and particularly to gels wherein the ionomer is proton-conducting, as well as to related products incorporating such gels and methods for producing the same.
2. Description of the Related Art
In general, ion-exchange materials have been shown to be useful for a broad range of applications, and may generally be categorized as either anion- or cation-exchange materials. Such materials have been used in fields such as chromatography, catalysis, electrochemical processes, the creation of super acids and super bases, and for the separation, concentration and/or purification of various ionic species.
One important application of ion-exchange materials is their use as electrolytes in electrochemical fuel cells. In such applications, the electrolyte commonly conducts protons, and thus may be characterized as a cation-exchange material. Such cation-exchange materials may typically constitute an organic polymer having acidic functional groups attached thereto. The acidic functional groups, in turn, may comprise corresponding cations. In the context of fuel cell electrolytes, protons are the more common cations.
When the electrolyte is incorporated into a membrane, the ion-exchange material is often referred to as a proton-exchange membrane (or xe2x80x9cPEMxe2x80x9d), and fuel cells incorporating such a membrane are referred to as xe2x80x9cPEM fuel cells.xe2x80x9d Cation-exchange materials may also be incorporated into PEM fuel cells in other forms, for example, as components in the catalyst layers or as electrode coatings.
In general terms, an electrochemical fuel cell functions by combining hydrogen, a suitable fuel and oxygen to produce electricity, heat and water. Fundamental components of PEM fuel cells include two electrodesxe2x80x94the anode and cathodexe2x80x94separated by the PEM. Each electrode is coated on one side with a thin layer of catalyst, with the PEM being xe2x80x9csandwichedxe2x80x9d between the two electrodes and in contact with the catalyst layers. Alternatively, one or both sides of the PEM may be coated with a catalyst layer, and the catalyzed PEM is then sandwiched between a pair of porous and electrically conductive electrode substrates. The anode/PEM/cathode combination is referred to as a membrane electrode assembly or xe2x80x9cMEA.xe2x80x9d A suitable fuel is one that dissociates into electrons and protons upon contact with the catalyst on the anode-side of the MEA. The protons migrate through the PEM, while the free electrons travel from the anode to the cathode, by way of an external circuit, producing a form of usable electric current. Upon contacting the catalyst on the cathode-side of the MEA, the protons that passed through the PEM, as well as oxygen and the electrons from the external circuit, combine to form water.
Desirable characteristics of a PEM include certain mechanical properties, high conductivity, resistance to oxidative and thermal degradation, and dimensional stability upon hydration and dehydration. It is also desirable to have a PEM with characteristics, including ease of handling, that allow it to be easily incorporated into a larger scale fabrication process. A variety of materials have been developed with these characteristics in mind, including perfluorinated sulfonic acid aliphatic polymers such as those described in U.S. Pat. Nos. 3,282,875 and 4,330,654. One example is a product sold by Dupont under the trademark Nafion(copyright), which is a polytetrafluoroethylene-based ionomer containing sulfonic acid groups to provide proton conductivity.
Nafion(copyright) solutions have been shown to be generally suitable for blending with various forms of raw catalyst to create catalyst inks that can be applied to the surface of anode and/or cathode electrodes. For instance, nominal 10% aqueous Nafion(copyright) solution and nominal 20% alcoholic Nafion(copyright) solution are available and have been found to be suitable for use in a catalyst ink. However, such solutions and the inks prepared from them are typically characterized by relatively low viscosities.
The method by which the catalyst ink is to be applied to the electrode also requires specific application characteristics. Until recently, spraying has been used as the primary method of applying the catalyst layer. Advances in direct methanol fuel cell (DMFC) technology have lead to an increased demand for DMFC electrodes. It has been proposed that larger scale fabrication processes that screen-print the catalyst layer may prove more useful. A catalyst ink used to spray DMFC high-loaded anodes, made from a process that utilizes a suspension of 5% Nafion(copyright) in 2-propanol/water, comprising a solids content of approximately 12%, which include Pt/Ru black, 11% Nafion(copyright) and water has previously been utilized. Although this ink has been shown to be useful for preparing catalyst layers via spraying, it has not been suitable for screen-printing.
Screen-printing inks are generally prepared in larger batches and are used over a longer period of time. These conditions make it necessary that inks be resistant to separation or settling of the catalyst out of suspension. Furthermore, ink for screen-printing must have the properties of substantial viscosity (xcx9c1000 centipoise or greater@shear rates of about 10 secondxe2x88x921), as well as both chemical and physical stability. For example, a continuous phase which is more viscous that the 5% Nafion(copyright) in 2-propanol/water previously used for spray application is necessary. Attempts to increase the ink viscosity, particularly utilizing aqueous Nafion(copyright) have been investigated. However, the previously attained viscosity of the aqueous suspension generally has not been adequate to suspend the catalyst. In addition, electrodes prepared with this ink have performed lower than the baseline spray techniques, particularly at high current densities (e.g.  greater than 200 mA/cm2) where performance is dominated by mass transport effects.
Accordingly, there remains a general need in the art for improved aqueous ionomer gels and, more particularly, for aqueous ionomer gels suitable for screen-printing electrodes of electrochemical fuel cells. The present invention fulfills these needs, and provides further related advantages.
In brief, an aqueous ionomer gel is disclosed that is substantially free of organic solvent(s), wherein the ionomer gel has an ionomer solids content ranging from about 4% to about 18% by weight of the ionomer gel, and a viscosity in excess of 5,000 centipoise at a shear rate of 10 secondsxe2x88x921. Suitable ionomers contain both a hydrophobic portion and an ionic portion and, in one embodiment, the ionomer is a graft copolymer having a hydrophobic backbone with pendent ionic portions grafted thereto. The ionomer may be a proton conducting ionomer, such as a perfluorosulfonate ionomer (i.e., Nafion(copyright)).
In a further embodiment, a catalyst ink is disclosed comprising an aqueous ionomer gel and a catalyst. Representative catalysts include, for example, noble metal catalysts including platinum. Such catalyst inks are suitable for coating a substrate surface in need of catalyst coatings, such as an electrode of an electrochemical fuel cell, particularly in the context of electrode screen-printing. Alternatively, such inks may be molded into various forms, such as a membrane or sheet, or may be coated onto a membrane, or may further comprise additional elements including a filler, binder and/or a pore forming material.
In other embodiments, methods are disclosed for making an aqueous ionomer gel. In one aspect, the method includes the steps of providing a solution comprising an ionomer, water and a nonaqueous solvent having a boiling point less than 100xc2x0 C., wherein the nonaqueous solvent is miscible with water; and evaporating the nonaqueous solvent below ambient pressure to produce the aqueous ionomer gel. This method may further include the step of cooling the aqueous ionomer gel. The solution comprising the ionomer, water and the nonaqueous solvent may be formed by addition of the nonaqueous solvent to an aqueous ionomer solution.
In another aspect, the method includes the steps of rapidly cooling an aqueous ionomer solution to form a substantially frozen form of the aqueous ionomer solution, and thawing the substantially frozen form of the aqueous ionomer solution to produce the aqueous ionomer gel. After the step of thawing, the resulting aqueous ionomer gel may be diluted by addition of water.
The methods further include the step of adding a catalyst to the resulting aqueous ionomer solution to form the catalyst ink, as well as the application of such catalyst ink to a substrate surface with an optional annealing step. Products made according to the methods of this invention are also disclosed.
These and other aspects of this invention will be evident upon references to the following detailed description and attached drawings.